1. Field of the Invention
The present invention relates to a fuel supply pump.
2. Description of Related Art
With reference to FIG. 4, one prior art fuel supply pump 100 includes a plunger 101, a cylinder body 103 and a plunger drive mechanism 105 (see, for example Japanese Unexamined Patent publication JP2010-144673A). The plunger 101 is adapted to axially reciprocate. The cylinder body 103 has a cylinder hole 102, in which the plunger 101 is received such that the plunger 101 is adapted to axially reciprocate in the cylinder hole 102. The plunger drive mechanism 105 converts rotational motion, which is transmitted from an internal combustion engine (not shown), into linear reciprocating motion and conducts it to the plunger 101. The fuel supply pump 100 is applied to an accumulator type fuel injection system, which injects the high pressure fuel of over, for example, 100 MPa received from a common rail 106 into the internal combustion engine.
In the prior art fuel supply pump 100, one axial end portion of the cylinder hole 102 is partitioned by the plunger 101 to form a pressurizing chamber 108. When the plunger 101 is axially reciprocated in the cylinder hole 102, a volume of the pressurizing chamber 108 is changed to draw fuel into the pressurizing chamber 108 and to discharge the fuel from the pressurizing chamber 108 upon pressurizing the same in the pressurizing chamber 108. Hereinafter, the plunger 101 and the cylinder body 103 may be collectively referred to as a high pressure pump 109.
In the prior art fuel supply pump 100, the fuel of the pressurizing chamber 108 leaks from the pressurizing chamber 108 to the other axial end side through a slide clearance 110 between the plunger 101 and the cylinder body 103. Therefore, a leak recovery groove 111 is formed between a slidable surface of the cylinder body 103 and a slidable surface of the plunger 101 by radially outwardly recessing a portion of the slidable surface of the cylinder body 103. The fuel of the leak recovery groove 111 is returned to a fuel tank 113 through a check valve 112.
Lately, the fuel injection pressure of the fuel injection system is increased to the high pressure, and there is a continuous demand to further increase the discharge pressure of the fuel supply pump 100. However, when the discharge pressure is further increased, the temperature of the fuel, which is compressed and pressurized in the pressurizing chamber 108, becomes higher. Therefore, the temperature of the high pressure pump 109 becomes high, and the following disadvantages may be encountered.
First of all, the fuel, which is drawn into the pressurizing chamber 108, is immediately heated by the heated high pressure pump 109, so that the temperature of the fuel becomes high, and thereby a viscosity of the fuel is reduced. Thus, the amount of fuel leakage from the pressurizing chamber 108 is increased to possibly cause a decrease in the discharge quantity of the fuel supply pump 100 (reduction of the discharge efficiency).
Furthermore, the amount of fuel leakage from the leak recovery groove 111 toward the other axial side may possibly be increased due to the decrease in the viscosity of the fuel of the pressurizing chamber 108. Therefore, in the case where the plunger drive mechanism 105 is lubricated by the lubricating oil, which has the viscosity higher than that of the fuel, the lubricating oil may possibly be diluted by the fuel to cause lubrication failure.
Furthermore, various seal elements, such as O-rings, are installed in the high pressure pump 109. The high temperature of the high pressure pump 109 may possibly cause deterioration of the lifetime of the seal elements.
Furthermore, the leaked fuel tends to be degraded by the high temperature, so that deposits may be accumulated in a return flow passage, which extends from the leak recovery groove 111 to the fuel tank 113, thereby possibly causing clogging of the flow passage with the accumulated deposits.
Therefore, it has been demanded to provide a structure of the fuel supply pump 100, which can limit the temperature increase of the high pressure pump 109 even upon increasing of the discharge pressure.